Composition changes after the 'Halloween' solar proton event: the High-Energy Particle Precipitation in the Atmosphere (HEPPA) model versus MIPAS data intercomparison study

We have compared composition changes of NO, NO<SUB>2</SUB>, H<SUB>2</SUB>O<SUB>2</SUB>, O<SUB>3</SUB>, N<SUB>2</SUB>O, HNO<SUB>3</SUB>, N<SUB>2</SUB>O<SUB>5</SUB>, HNO<SUB>4</SUB>, ClO, HOCl, and ClONO<SUB>2</SUB> as observed by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat in the aftermath of the 'Halloween' solar proton event (SPE) in October/November 2003 at 25-0.01 hPa in the Northern Hemisphere (40-90° N) and simulations performed by the following atmospheric models: the Bremen 2d Model (B2dM) and Bremen 3d Chemical Transport Model (B3dCTM), the Central Aerological Observatory (CAO) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, the modeling tool for SOlar Climate Ozone Links studies (SOCOL and SOCOLi), and the Whole Atmosphere Community Climate Model (WACCM4). The large number of participating models allowed for an evaluation of the overall ability of atmospheric models to reproduce observed atmospheric perturbations generated by SPEs, particularly with respect to NO<SUB>y</SUB> and ozone changes. We have further assessed the meteorological conditions and their implications on the chemical response to the SPE in both the models and observations by comparing temperature and tracer (CH<SUB>4</SUB> and CO) fields. <BR /><BR /> Simulated SPE-induced ozone losses agree on average within 5% with the observations. Simulated noy enhancements around 1 hPa, however, are typically 30% higher than indicated by the observations which can be partly attributed to an overestimation of simulated electron-induced ionization. The analysis of the observed and modeled NO<SUB>y</SUB> partitioning in the aftermath of the SPE has demonstrated the need to implement additional ion chemistry (HNO<SUB>3</SUB> formation via ion-ion recombination and water cluster ions) into the chemical schemes. An overestimation of observed H<SUB>2</SUB>O enhancements by all models hints at an underestimation of the OH/HO<SUB>2</SUB> ratio in the upper polar stratosphere during the SPE. The analysis of chlorine species perturbations has shown that the encountered differences between models and observations, particularly the underestimation of observed ClONO<SUB>2</SUB> enhancements, are related to a smaller availability of ClO in the polar night region already before the SPE. In general, the intercomparison has demonstrated that differences in the meteorology and/or initial state of the atmosphere in the simulations causes a relevant variability of the model results, even on a short timescale of only a few days.